What the achievement asks
Win an Island Expedition to Crestfall on Mythic difficulty. Ten points, both factions.
This one is not tied to your account. Each of the eleven island achievements has to be earned by the character that runs it, so switching alts between islands leaves you with nothing.
You can do it alone
The single most useful thing to know about Mythic islands is that solo is allowed, and almost nobody realises it.
There are two ways to queue, standing in the same spot. The table is the old group finder from Battle for Azeroth, and on Mythic it refuses to start without three players. The person standing behind the table queues you on your own.
Talk to Flynn Fairwind in Boralus if you are Alliance, or Captain Rez'okun in Dazar'alor if you are Horde, and pick Mythic from his list rather than using the table beside him.
At current levels a solo Mythic island is not a difficult fight. It is a race.
What winning means
An expedition is a contest against a rival team for azerite, and it ends when someone reaches the target, around twelve thousand. Killing things faster than the other side is the whole game.
Three things move the bar quickest:
- Packs led by an elite. They are worth far more than ordinary mobs, so go out of your way for them rather than clearing everything in sight.
- The rival team itself. Killing the opposing three is a large single jump. They will often come looking for you anyway.
- The bonus quests. They show as a yellow mark on the map, and the item they want is almost always right next to the marker and glowing.
The catch is the weekly rotation
Only three islands are offered in any given week, and you do not get to choose which. Crestfall will not be on the list most weeks.
That is what makes the eleven island achievements slow rather than hard. Each is one win, but you can only chase the ones currently in rotation, so the set takes at least four weeks and usually longer. Check the list each week and take whatever is up.
Running these anyway feeds Expert Expeditioner, which wants fifty Mythic wins. Since you will be queueing repeatedly for the rotation regardless, it is worth doing the two together rather than farming twice.